In recent years, projection lithographic technology using EUV light having a shorter wavelength (11 to 14 nm) than that of conventional ultraviolet light has been developed to enhance the resolution of optical systems that are limited by the diffraction limit of light accompanying miniaturization of semiconductor integrated circuits. This technology in recent years has been called EUV (Extreme Ultraviolet) lithography. It is expected to be technology capable of providing resolution under 70 nm that had been impossible with conventional optical lithography using a light source of 190 nm wavelength.
The complex refractive index n of substance in the wavelength range of EUV light is represented by n=1− c− ik (i is the complex notation). Imaginary part k in this refractive index represents extreme ultraviolet absorption. Since c is far smaller than 1, the real part of the refractive index in this range is very close to 1. Furthermore, since k has a finite value in all substances, absorption must take place. Accordingly, an optical system that uses reflection is used since a transparent refractive optical component like a conventional lens cannot be used.
FIG. 11 presents an outline of the EUV exposure apparatus. EUV light 32 discharged from EUV light source 31 is incident to illumination optical system 33. It forms a virtually parallel luminous flux via concave mirror 34 that acts as a collimator mirror. It is then incident to optical integrator 35 comprising a pair of fly-eye mirrors 35a and 35b. The fly-eye mirror disclosed in the gazette of Japanese Kokai Publication Hei-11-312638, for example, can be used as the pair of fly-eye mirrors 35a and 35b. The detailed structure and function of the fly-eye mirror are explained in further detail in the gazette of Japanese Kokai Publication Hei-11-312638, and their explanation is omitted here since it is unrelated directly to the present invention.
A surface illuminant having a predetermined shape is formed near the reflecting surface of second fly-eye mirror 35b, specifically, near the emission surface of optical integrator 35. Light from the surface illuminant is polarized by plate mirror 36, followed by formation of an elongated, arc shaped illumination region on mask M (a depiction of the perforated plate used to form the arc shaped illumination region is omitted). Light from the pattern on mask M forms an image of the mask pattern on wafer W via projection optical system PL comprising a plurality of mirrors (six mirrors M1 to M6 shown in FIG. 11).
The optical system using such mirrors has a ring-shaped projection exposure field in which only aberration at a specific image height is compensated since it cannot compensate for overall aberration in the broad exposure field. The mask and wafer are scanned in synchronization and exposed since a 30 mm square chip cannot be exposed at once in a projection exposure field having such a ring shape.
A multi-layer film reflecting mirror is generally used as the reflecting mirror in such an EUV exposure apparatus. A multi-layer film is formed on a substrate, and the phase of very weak reflecting light at the interface is aligned and multiply overlapped to obtain high reflectance.
Reflectance of 67.5% at perpendicular incidence can be attained by using an Mo/Si multi-layer film in which a molybdenum (Mo) layer and a silicon (Si) layer are alternately laminated in the wavelength region near 13.4 nm while reflectance of 70.2% at perpendicular incidence can be attained by using an Mo/Be multi-layer film in which an Mo layer and a beryllium (Be) layer are alternately laminated in the wavelength region near 11.3 nm.
In light source which is commonly used as EUV light source 31, laser light is irradiated on target material as excitation light to convert the target material into plasma. The EUV light (exposure light) generated at that time is then used. Such an EUV light source is discussed in the gazette of Japanese Kokai Publication 2000-56099.
The use of Xe plasma (as both the laser plasma light source and the discharge plasma light source) has been extensively researched and developed as the EUV light source of 13.5 nm wavelength used in an EUV exposure apparatus. The reasons are that a comparatively high conversion efficiency (proportion of EUV light intensity attained relative to input energy) can be attained and that no problems arise associated with debris since the material is gaseous at ambient temperature. However, there are limits to how high a conversion efficiency can be realized because Xe is a gas, and the use of Sn as target material is known to be effective in attaining a higher conversion efficiency.
There is an electron temperature of ideal plasma for efficient generation of EUV light, and 50 eV is ideal. The electron temperature rises with increase in the irradiation intensity of laser light when the light source is laser plasma. The electron temperature rises excessively and X-rays of short wavelength are generated when the irradiation intensity is too high, thereby lowering the conversion efficiency from laser light to EUV light. Accordingly, there is an ideal laser irradiation intensity, which is about 1011 W/cm2.
The laser spot diameter must be increased to elevate the EUV light output while retaining the ideal light irradiation intensity.
The size of the EUV light source is limited by the etendue (product of the cross-sectional area of a luminous flux and the solid angle). Etendue is the quantity retained in an optical system. When the product of the size (area) of the light source and the capture solid angle of a convergence optical system exceeds the etendue of a convergence optical system, the excess portion of EUV light cannot be captured in the optical system and is wasted. Consequently, a maximum allowable level of the size of the light source exists.
Accordingly, setting the light spot diameter to the maximum allowable level would be effective in maximumizing the output of an effective EUV light. For example, if the numerical aperture (NA) of a projection optical system were 0.25 (sin θ=0.25, or 0.2 sr when converted to a solid angle), the exposure field size were 2 mm×25 mm, and the δ value of illumination were 0.5, the etendue of this optical system would be 2×25×0.2×0.5=5 mm2sr.
Assuming the capture solid angle of the convergence optical system to be π and the allowable light source size to be 5/π=1.6 mm2, then the maximum allowable diameter of the light source would be 1.4 mm.
On the other hand, debris is a serious problem in an EUV light source using plasma. The reflectance falls markedly when debris adheres to the surface of a condenser mirror. Typical debris includes fragments of target material, large particles that solidify after once dissolution, ions generated in plasma, and neutral particles of atomic shape that have lost their charge due to the charge exchange collision of ions, and debris is found in various sizes.
The so-called limited mass target is an effective method of inhibiting debris comprising fragments of target material and large particles that solidify after once dissolution. This is a method that uses minimum material required for generating plasma. If all target material could be converted into plasma (ionized), the debris that was finely decomposed into atomic shape, including the neutral particles of atomic shape that have lost their charge due to the charge exchange collision of ions as well as ions generated in plasma, could be removed from the optical path by such aspects as a gas curtain, while debris of atomic shape that has a charge could be removed from the optical path by an electromagnetic field.
Examination of the thickness of target material required to form plasma following irradiation of laser light on a target confirmed a thickness under 100 nm to be adequate. Consequently, the providing of extremely flat target material of 100 nm thickness with a diameter of 1 mm (a value smaller than the 1.4 mm maximum diameter of the allowable light source) that is perpendicular to the incident direction of laser light would be effective in maximizing the EUV output while holding down the amount of debris generation in a laser plasma light source.
However, nothing that satisfies these requirements has been available in target providing methods considered to date. At present, the jet target method and the droplet target method are the methods of providing targets considered to be effective in inhibiting debris.
The jet target method is a aspect in which liquid target material is continuously sprayed in a vacuum from a nozzle to continuously provide an elongated columnar target that is solidified instantly due to adiabatic expansion. The droplet target method is a aspect in which target material is continuously sprayed from a nozzle to continuously provide target material that is spherical due to surface tension.
The target dimensions of these did not differ between the irradiation direction of laser light and the perpendicular direction in either target method, and the extremely flat ideal target shape was not obtained. Accordingly, the entire target could not be converted into plasma and the problems of residue generation due to debris could not be avoided.